We propose to investigate the mechanisms by which eicosopentaenoic acid (EPA) and docosohexaenoic acid (DHA) act to reduce plasma concentrations of triglyceride (TG) and cholesterol (C). Our approach to the problem will be to study the hepatic metabolism of fatty acids, the formation and secretion of the very low density lipoprotein (VLDL), its relationship to cholesterogenesis, and the effects of omega-3-polyunsaturated fatty acids on these processes. Experiments will be carried out in model systems in vitro using the rat as an experimental animal, but will be correlated with complementary in vivo studies. Using isolated perfused livers obtained from normal chow fed rats, we will evaluate the uptake of EPA and DHA, the output of TG, and rates of oxidation, compared to other fatty acids. The total disposition of (U-14C)EPA and DHA among metabolic pathways will be determined. The possibility that EPA and DHA might alter the hepatic metabolism of the common mammalian fatty acids will be investigated. The effects of EPA/DHA on output of VLDL lipids, the lipid and apoprotein composition of the VLDL, the synthesis and secretion of VLDL apoproteins, and the characterization of the VLDL in the zonal ultracentrifuge will be studied in detail under conditions described above. Since fatty acids stimulate hepatic secretion and denovo synthesis of C, the effects of EPA and DHA on these parameters will be investigated. Furthermore, the effects of EPA/DHA on the coordinate stimulation by fatty acids of enzymes of cholesterogenesis (HMG- CoA reductase, HMG-CoA synthase, AcAc CoA thiolase) will be investigated, as will any intrinsic activity of the omega-3-fatty acids. These parameters will also be evaluated in livers obtained from animals on diets high in EPA, compared with diets supplemented with lard or olive oil. Furthermore, we will correlate these dietary studies with estimation of hepatic secretion of VLDL in vivo. As adjuncts to our primary studies with the perfused liver, we will evaluate fatty acid metabolism, with emphasis on synthesis of triglyceride, by isolated hepatocytes and microsomal preparations. These comparative experiments will help us evaluate the efficacy of EPA/DHA as substrates for TG synthesis, as well as putative competitive inhibitors of other common fatty acid substrates. The extraordinary effects of omega-3-unsaturated fatty acids to lower plasma TG and C, the obvious clinical relevance of these observations, and the probability that they act through regulation of VLDL synthesis and secretion form the basis of this proposal.